U.S. patent number 6,483,826 [Application Number 09/252,990] was granted by the patent office on 2002-11-19 for utilization of plural multiple access types for mobile telecommunications.
This patent grant is currently assigned to Telefonaktiebolaget LM Ericsson (publ). Invention is credited to Dag E:son .ANG.kerberg.
United States Patent |
6,483,826 |
.ANG.kerberg |
November 19, 2002 |
Utilization of plural multiple access types for mobile
telecommunications
Abstract
A first telecommunications system (20.sub.1) has at least
partially geographically overlapping coverage with a second
telecommunications system (20.sub.2), with radio transmission
access in the first telecommunications system utilizing code
division multiple access (CDMA) with a time division multiple
access (TDMA) component. A base station (BS) of the first
telecommunications system communicates over an air interface with a
subscriber station (SS) and broadcasts a common broadcast physical
channel (BPCH) on at least an active carrier/timeslot combination
(C/TS). The common broadcast physical channel includes broadcast
system identity information. The subscriber station uses the
broadcast system identity information to distinguish between
transmissions of the first telecommunications system and the second
telecommunications system. Moreover, the subscriber station uses
the broadcast system identity information for framing transmissions
in the first telecommunications system. The base station of the
first system broadcasts at least two common broadcast physical
channels active on different timeslots. Preferably, a fixed direct
sequence CDMA code is employed for the common broadcast physical
channel. In accordance with an instant dynamic channel selection
procedure of the present invention, the subscriber station selects
a carrier/timeslot for sending an access request to the first
system base station. The carrier/timeslot selected by the first
system subscriber station for sending an access request to the
first system base station is also used for a traffic channel.
Preferably, the first system subscriber station selects a least
interfered carrier/timeslot as the carrier/timeslot for sending an
access request to the first system base station. In response to the
access request, the first system base station sends an answer on a
next down-link carrier/timeslot included in a duplex pair with the
carrier/time slot which the first system subscriber station
selected for sending the access request to the first system base
station. At an given moment a connection between the first system
base station and the subscriber station is borne on a physical
channel of the first telecommunications system, the physical
channel being defined by a code, timeslot, and frequency.
Inventors: |
.ANG.kerberg; Dag E:son
(Kungsangen, SE) |
Assignee: |
Telefonaktiebolaget LM Ericsson
(publ) (Stockholm, SE)
|
Family
ID: |
22958389 |
Appl.
No.: |
09/252,990 |
Filed: |
February 19, 1999 |
Current U.S.
Class: |
370/335; 370/337;
455/422.1 |
Current CPC
Class: |
H04W
48/12 (20130101); H04B 1/707 (20130101); H04W
68/00 (20130101); H04W 74/0866 (20130101) |
Current International
Class: |
H04Q
7/38 (20060101); H04J 013/00 () |
Field of
Search: |
;370/335,342,320,337,347,321 ;455/62,63,422,476,446,561 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
0 652 680 |
|
May 1995 |
|
EP |
|
07/36443 |
|
Oct 1997 |
|
WO |
|
Other References
ETR 310: Annex E: DECT Instance DCS Procedures, Aug. 1996, pp.
69-74. .
.ANG.kerberg, Novel Radio Access Principles Useful for Third
Generation Mobile Radio Systems, PIMRC '92, the Third IEEE
International Symposium on Personal, Indoor and Mobile Radio
Communications Proceedings, Oct. 19-21, 1992, pp. 4-9..
|
Primary Examiner: Kincaid; Lester G.
Attorney, Agent or Firm: Nixon & Vanderhye, P.C.
Claims
What is claimed is:
1. A telecommunications system comprising at least one base station
which communicates over an air interface with a subscriber station,
wherein radio transmission access in the telecommunications system
utilizes code division multiple access (CDMA) with a time division
multiple access (TDMA) component; wherein a common broadcast
physical channel is broadcast from the base station at least on
each active carrier/timeslot combination; and wherein the common
broadcast physical channel includes (1) broadcast system identity
information and (2) paging information.
2. The system of claim 1, wherein for the base station there are
always at least two common broadcast physical channels active on
different timeslots.
3. The system of claim 2, wherein the base station creates a dummy
common broadcast physical channel on a least interfered
timeslot.
4. The system of claim 1, wherein the base station puts the two
common broadcast physical channels on active traffic timeslots, but
if there are less than two active traffic timeslots the base
station creates a dummy common broadcast physical channel on a
least interfered non-traffic timeslot.
5. The system of claim 3, wherein the base station monitors to
ensure that the dummy common broadcast physical channel remains on
a least interfered timeslot, and if not relocates the dummy common
broadcast physical channel to the least interfered timeslot.
6. The system of claim 1, wherein a fixed direct sequence CDMA code
is employed for the common broadcast physical channel.
7. The system of claim 1, wherein the subscriber station selects a
carrier/timeslot for sending an access request to the base
station.
8. The system of claim 7, wherein the carrier/timeslot selected by
the subscriber station for sending an access request to the base
station is also used for a traffic channel.
9. The system of claim 7, wherein the subscriber station selects a
least interfered carrier/timeslot as the carrier/timeslot for
sending an access request to the base station.
10.The system of claim 9, wherein the broadcast system identity
further includes an identification of the base station of the
telecommunications system to which the subscriber belongs.
11. The system of claim 7, wherein upon receipt of the access
request, for an accepted access request the base station sends an
answer on a next down-link carrier/timeslot included in a duplex
pair with the carrier/time slot which the subscriber station
selected for sending the access request to the base station.
12. The system of claim 7, wherein the subscriber station selects a
least interfered uplink carrier/timeslot as the carrier/timeslot
for sending an access request to the base station must be above a
first predetermined threshold, and wherein a duplex paired downlink
carrier/timeslot for the selected uplink carrier/timeslot must be
above a second predetermined threshold as measured at the base
station.
13. The system of claim 12, wherein in selecting the least
interfered uplink carrier/timeslot as the carrier/timeslot for
sending an access request to the base station, the subscriber
station utilizes the following priority: (1) a duplex
carrier/timeslot channel having a dummy common broadcast channel;
(2) a duplex carrier/timeslot channel having an active traffic
channel of the base station closest to the subscriber station; (3)
a duplex carrier/timeslot channel having an active traffic channel
of a neighboring base station a least interfered duplex
carrier/timeslot channel on a random carrier.
14. The system of claim 1, wherein at an given moment a connection
between the base station and the subscriber station is borne on a
physical channel of the system, the physical channel being defined
by a code, timeslot, and frequency.
15. The system of claim 1, wherein the broadcast system identity
includes information which enables the subscriber station
distinguish the telecommunications system to which the subscriber
belongs from any other telecommunications system operating in a
same geographical area, and whereby the subscriber station uses the
broadcast system identity for framing transmissions from the
telecommunications system to which the subscriber belongs.
16. A method of operating a telecommunications system comprising at
least one base station which communicates over an air interface
with a subscriber station, the method comprising: using code
division multiple access (CDMA) with a time division multiple
access (TDMA) component for radio transmission access in the
telecommunications system; broadcasting a common broadcast physical
channel from the base station at least on each active
carrier/timeslot combination; and including in the common broadcast
physical channel (1) broadcast system identity information and (2)
paging information.
17. The method of claim 16, further comprising maintaining, for the
base station, at least two common broadcast physical channels
active on different timeslots.
18. The method of claim 17, further comprising the base station
putting the two common broadcast physical channels on active
traffic timeslots, but if there are less than two active traffic
timeslots the base station creates a dummy common broadcast
physical channel on a least interfered non-traffic timeslot.
19. The method of claim 18, further comprising the base station
monitoring to ensure that the dummy common broadcast physical
channel remains on a least interfered timeslot, and if not
relocating the dummy common broadcast physical channel to the least
interfered timeslot.
20. The method of claim 16, further comprising the base station
creating a dummy common broadcast physical channel on a least
interfered timeslot.
21. The method of claim 16, further comprising employing a fixed
direct sequence CDMA code for the common broadcast physical
channel.
22. The method of claim 16, further comprising the subscriber
station selecting a carrier/timeslot for sending an access request
to the base station.
23. The method of claim 22, further comprising also using, for a
traffic channel, the carrier/timeslot selected by the subscriber
station for sending an access request to the base station.
24. The method of claim 22, further comprising the subscriber
station selecting a least interfered carrier/timeslot as the
carrier/timeslot for sending an access request to the base
station.
25. The method of claim 24, wherein the subscriber station
selecting a least interfered uplink carrier/timeslot as the
carrier/timeslot for sending an access request to the base station
includes requiring the selected carrier/timeslot being above a
first predetermined threshold, and wherein a duplex paired downlink
carrier/timeslot for the selected uplink carrier/timeslot must be
above a second predetermined threshold as measured at the base
station.
26. The method of claim 25, wherein in the step of selecting the
least interfered uplink carrier/timeslot as the carrier/timeslot
for sending an access request to the base station, the subscriber
station utilizes the following priority: (1) a duplex
carrier/timeslot channel having a dummy common broadcast channel;
(2) a duplex carrier/timeslot channel having an active traffic
channel of the base station closest to the subscriber station; (3)
a duplex carrier/timeslot channel having an active traffic channel
of a neighboring base station a least interfered duplex
carrier/timeslot channel on a random carrier.
27. The method of claim 22, further comprising sending from the
base station, upon receipt of the accepted access request, an
answer on a next down-link carrier/timeslot included in a duplex
pair with the carrier/time slot which the subscriber station
selected for sending the access request to the base station.
28. The method of claim 16, wherein at an given moment a connection
between the base station and the subscriber station is borne on a
physical channel of the system, the physical channel being defined
by a code, timeslot, and frequency.
29. The method of claim 16, further comprising the subscriber
station using the broadcast system identity information to
distinguish the telecommunications system to which the subscriber
belongs from any other telecommunications system operating in a
same geographical area.
30. The method of claim 29, further comprising the subscriber
station using the broadcast system identity information for framing
transmissions in the telecommunications system to which the
subscriber belongs.
31. A method of operating a first telecommunications system which
has at least partially geographically overlapping coverage with a
second telecommunications system, the first telecommunications
system having at least one first system base station which
communicates over an air interface with a first system subscriber
station, wherein radio transmission access in the first
telecommunications system utilizes code division multiple access
(CDMA) with a time division multiple access (TDMA) component; the
method comprising: broadcasting a common broadcast physical channel
from the first system base station at least on an active
carrier/timeslot combination, the common broadcast physical channel
including broadcast system identity information; and using the
broadcast system identity information at the first system
subscriber station to distinguish between transmissions of the
first telecommunications system and the second telecommunications
system.
32. The method of claim 31, further comprising using the broadcast
system identity information at the first system subscriber station
for framing transmissions in the first telecommunications
system.
33. The method of claim 31, further comprising further including
paging information in the common broadcast physical channel.
34. The method of claim 33, further comprising the first system
base station creating a dummy common broadcast physical channel on
a least interfered timeslot.
35. The method of claim 31, further comprising maintaining, for the
first system base station, at least two common broadcast physical
channels active on different timeslots.
36. The method of claim 35, further comprising the first system
base station putting the two common broadcast physical channels on
active traffic timeslots, but if there are less than two active
traffic timeslots the base station creates a dummy common broadcast
physical channel on a least interfered non-traffic timeslot.
37. The method of claim 36, further comprising the first system
base station monitoring to ensure that the dummy common broadcast
physical channel remains on a least interfered timeslot, and if not
relocating the dummy common broadcast physical channel to the least
interfered timeslot.
38. The method of claim 31, further comprising employing a fixed
direct sequence CDMA code for the common broadcast physical
channel.
39. The method of claim 31, further comprising the first system
subscriber station selecting a carrier/timeslot for sending an
access request to the first system base station.
40. The method of claim 31, further comprising also using, for a
traffic channel, the carrier/timeslot selected by the first system
subscriber station for sending an access request to the first
system base station.
41. The method of claim 40, further comprising the first system
subscriber station selecting a least interfered carrier/timeslot as
the carrier/timeslot for sending an access request to the first
system base station.
42. The method of claim 40, further comprising sending from the
first system base station, upon receipt of the accepted access
request, an answer on a next down-link carrier/timeslot included in
a duplex pair with the carrier/time slot which the first system
subscriber station selected for sending the access request to the
first system base station.
43. The method of claim 31, wherein at an given moment a connection
between the first system base station and the first system
subscriber station is borne on a physical channel of the first
telecommunications system, the physical channel being defined by a
code, timeslot, and frequency.
44. A first telecommunications system which has at least partially
geographically overlapping coverage with a second
telecommunications system, wherein radio transmission access in the
first telecommunications system utilizes code division multiple
access (CDMA) with a time division multiple access (TDMA)
component; the first telecommunications system comprising: the
first telecommunications system having at least one first system
base station which communicates over an air interface with a first
system subscriber station, the first system base station
broadcasting a common broadcast physical channel on at least an
active carrier/timeslot combination, the common broadcast physical
channel including broadcast system identity information; and a
first system subscriber station which uses the broadcast system
identity information to distinguish between transmissions of the
first telecommunications system and the second telecommunications
system.
45. The apparatus of claim 44, wherein the first system subscriber
station uses the broadcast system identity information framing
transmissions in the first telecommunications system.
46. The apparatus of claim 44, wherein the first system base
station further includes paging information in the common broadcast
physical channel.
47. The apparatus of claim 44, wherein the first system base
station broadcasts at least two common broadcast physical channels
active on different timeslots.
48. The apparatus of claim 47, wherein the first system base
station puts the two common broadcast physical channels on active
traffic timeslots, but if there are less than two active traffic
timeslots the base station creates a dummy common broadcast
physical channel on a least interfered non-traffic timeslot.
49. The apparatus of claim 48, wherein the first system base
station monitors to ensure that the dummy common broadcast physical
channel remains on a least interfered timeslot, and if not
relocates the dummy common broadcast physical channel to the least
interfered timeslot.
50. The apparatus of claim 44, wherein the first system base
station creates a dummy common broadcast physical channel on a
least interfered timeslot.
51. The apparatus of claim 44, wherein a fixed direct sequence CDMA
code is employed for the common broadcast physical channel.
52. The apparatus of claim 44, wherein the first system subscriber
station selects a carrier/timeslot for sending an access request to
the first system base station.
53. The apparatus of claim 52, wherein the carrier/timeslot
selected by the first system subscriber station for sending an
access request to the first system base station is also used for a
traffic channel.
54. The apparatus of claim 52, wherein the first system subscriber
station selects a least interfered carrier/timeslot as the
carrier/timeslot for sending an access request to the first system
base station.
55. The apparatus of claim 52, wherein the first system base
station sends, upon receipt of the accepted access request, an
answer on a next down-link carrier/timeslot included in a duplex
pair with the carrier/time slot which the first system subscriber
station selected for sending the access request to the first system
base station.
56. The apparatus of claim 52, wherein at an given moment a
connection between the first system base station and the first
system subscriber station is borne on a physical channel of the
first telecommunications system, the physical channel being defined
by a code, timeslot, and frequency.
57. A telecommunications system comprising at least one base
station which communicates over an air interface with a subscriber
station, wherein radio transmission access in the
telecommunications system utilizes code division multiple access
(CDMA) with a time division multiple access (TDMA) component;
wherein a duplex channel comprises an uplink carrier/timeslot
combination paired with a downlink carrier/timeslot combination
with a defined time relation, wherein a carrier/timeslot
combination utilized by the subscriber station for an access
request and the carrier/timeslot combination utilized by the base
station for the access request response shall utilize the same
duplex channel as the following uplink and downlink dedicated
physical channels used for a connection between the subscriber
station and the base station, and wherein a CDMA code is employed
for the dedicated physical channels.
58. The system of claim 57, wherein the access request and the
access request response are encoded with a fixed spreading
code.
59. The system of claim 57, wherein the dedicated physical channels
are encoded with a spreading code native to the subscriber
station.
60. The system of claim 57, wherein the carrier/timeslot
combination utilized by the base station for access request
response also carries a common broadcast physical channel, and
wherein the common broadcast physical channel includes (1)
broadcast system identity information and (2) paging
information.
61. The system of claim 60, wherein a CDMA code is employed for the
common broadcast physical channel.
62. A method of operating a telecommunications system comprising at
least one base station which communicates over an air interface
with a subscriber station, the method comprising: the subscriber
station sending an access request to the base station; the base
station sending an access request response to the subscriber
station; using code division multiple access (CDMA) with a time
division multiple access (TDMA) component for radio transmissions
in the telecommunications system; wherein a carrier/timeslot
combination utilized by the subscriber station for the access
request and a carrier/timeslot combination utilized by the base
station for the access request response are also utilized
respectively as uplink and downlink dedicated physical channels for
a connection between the subscriber station and the base station;
and wherein a CDMA code is employed for the dedicated physical
channels.
63. The method of claim 62, further comprising encoding the access
request and the access request response with a fixed spreading
code.
64. The method of claim 62, further comprising encoding the
dedicated physical channels with a spreading code native to the
subscriber station.
65. The method of claim 62, further comprising carrying on the
carrier/timeslot combination utilized by the base station for
access request response a common broadcast physical channel, the
common broadcast physical channel including (1) broadcast system
identity information and (2) paging information.
66. The system of claim 65, wherein a CDMA code is employed for the
common broadcast physical channel.
Description
BACKGROUND
1. Field of the Invention
The present invention pertains to wireless or mobile
telecommunications system which utilize multiple access type
technologies, and particularly to a channel selection scheme for a
multiple technology access type system, e.g., CDMA/TDMA system.
2. Related Art and Other Considerations
In wireless or mobile telecommunications systems, user equipment in
the form of a mobile station (e.g., mobile telephone) is typically
in radio communication over an air interface with a base station.
There are numerous techniques for providing access to the mobile
telecommunications system for multiple users, e.g., multiple mobile
stations. One general technique (known as frequency division
multiple access [FDMA]) employs plural carrier radio frequencies,
and assigns each user a different radio frequency. But since each
simultaneous radio frequency transmission requires one radio, other
techniques are often utilized to decrease cost and increase the
flexibility of offered services (either alone or in conjunction
with FDMA). One such other technique is time division multiple
access [TDMA], wherein each carrier frequency is conceptualized as
carrying frames of information, each frame then being divided into
time slots or channels, with differing users having differing time
slots. Typically (for duplex) there needs to be an "uplink" time
slot or channel for sending traffic from the mobile to the base
station and a "downlink" time slot for sending traffic from the
base station to the mobile.
One example technology which combines FDMA and TDMA (multi-carrier
TDMA [MC/TDMA]) operates in Europe in the spectrum of 1880-1900 MHz
under the name DECT. Aspects of DECT are described in ETR 310:
August 1996, Annex E: DECT Instant Dynamic Channel Selection (DCS)
Procedures, pp. 69-74. DECT installations are primarily
intra-company mobile telecommunications systems which typically are
quite limited in geographical range. In DECT, mobile stations are
always locked to a closest (strongest) base station. After having
locked itself to a strongest of the base stations, the mobile
station makes a list of least interfered channels which it
regularly updates. When the user of the mobile station desires to
make a call, the mobile station selects the best channel and sends
an access request (RACH) message to the strongest base station.
This request is sent in synchronism with the base station receiver
RF carrier scanning order. If a response is received on the
relevant duplex response slot, the duplex channel is established.
In DECT, handover is controlled by the mobile station. An automatic
or seamless ("make before break") handover is made as soon as
another base station becomes stronger. DECT provides paging and
system information on every downlink channel.
Mobile telecommunications service can be provided using yet another
multiple access technique known as code division multiple access
(CDMA). In a code division multiple access (CDMA) mobile
telecommunications system, the information transmitted between a
base station and a particular mobile station is modulated by a
mathematical code (such as spreading code) to distinguish it from
information for other mobile stations which are simultaneously
utilizing the same radio frequency. Thus, in CDMA, the individual
radio links are discriminated on the basis of codes. In addition,
in CDMA mobile communications, typically the same baseband signal
with suitable spreading is sent from several base stations with
overlapping coverage. The mobile terminal can thus receive and use
signals from several base stations simultaneously. Moreover, since
the radio environment changes rapidly, a mobile station likely has
radio channels to several base stations at the same moment, e.g.,
so that the mobile station can select the best channel and, if
necessary, use signals directed to the mobile from various base
stations in order to keep radio interference low and capacity high.
This utilization of radio channels to/from multiple base stations
by a mobile station, such as occurs in a CDMA scheme for example,
is termed "soft handover" or "macro diversity." In some
geographical areas, FDMA/CDMA or MC/CDMA is utilized in the
frequency spectrum beginning at 1920 MHz. Various aspects of CDMA
are set forth in Garg, Vijay K. et al., Applications of CDMA in
Wireless/Personal Communications, Prentice Hall (1997).
Traditional Direct Sequence CDMA (DS-CDMA) mobile radio systems
require careful coordination of all systems using a common
spectrum. DS-CDMA systems require rapid and accurate up-link power
control of all mobile stations within a cell. Accordingly,
operation of geographically overlapping DS-CDMA system is
problematic. Thus, typically DS-CDMA does not have more than one
system controlling the transmit power of mobile stations within a
given cell and same spectrum.
Subject to the foregoing concerns, if overlapping DS-CDMA systems
were configured and discriminated on the basis of spreading codes,
the systems likely would not have sufficient capacity. In this
regard, while a first DS-CDMA system might be assigned a first
subset of the spreading codes and a second DS-CDMA system be
assigned a second subset of the spreading codes, it should be kept
in mind that generally the number of usable codes is relatively
small (on the order of 50-100, for example). Moreover, this
constraint is particularly onerous if more than two DS-CDMA systems
were to overlap, or if expansion of further DS-CDMA systems is
desired in the same geographical region. Traditional DS-CDMA mobile
radio systems need careful coordination of all systems using a
common spectrum, since different systems (without handover between
them) can not operate in the same geographical area. The reason is
that the key functionality of DS-CDMA systems is based upon equal
down-link power for all connections and quick and accurate up-link
power control of all handsets within a cell. Therefore, it is not
allowed (on the same frequency) within a cell of a system to have
one or more handsets, whose transmit power is not under the control
of that system. This is in popular terms called the near/far
problem. Therefore, in order to accommodate a large number of
mobile stations, e.g., in a large geographical area, system
discrimination merely on the basis of spreading codes is
inadequate.
Opportunities exist for development of other segments of the
frequency spectrum, such as 1900-1920 MHz in Europe, for example.
What is particularly desired is to have plural uncoordinated
DECT-like systems operating in an overlapping fashion, e.g., in
larger than traditional DECT-sized geographical areas. It is
contemplated that future technologies might employ a marriage of
all three techniques: TDMA, FDMA, and CDMA. One main advantage of
DS-CDMA systems, compared to MC/TDMA systems, is that it is
possible to make a trade off between range and traffic capacity.
Yet is not clear how these three techniques can interwork
effectively, particularly since the sharing of spectrum resources
between uncoordinated systems installed in a same geographical area
is possible only in the frequency and time domains (but not the
code domain due, e.g., to the above-mentioned near/far
problem).
Traditional direct sequence DS-CDMA mobile radio systems need
careful coordination of all systems using a common spectrum, since
different systems (without handover between them) can not operate
in the same geographical area. The reason is that the key
functionality of DS-CDMA systems is based upon equal down-link
power for all connections and quick and accurate up-link power
control of all handsets within a cell. Therefore, it is not allowed
(on the same spectrum) within a cell of a system to have one or
more handsets, whose transmit power is not under the control of
that system.
Application of various kinds of Instant Dynamic Channel Selection
procedures (where the channel selection is made on per-call bases)
are known, e.g. for MC/TDMA systems like DECT, which provides for
uncoordinated system installations on a common spectrum. IDCS for
DS-CDMA does however require specific solutions to address the new
problems due to addition of CDMA. One main advantage of DS-CDMA
systems, compared to MC/TDMA systems, is that it is possible to
make a trade off between range and traffic capacity.
What is needed, therefore, and an object of the present invention,
is a channel selection scheme for uncoordinated systems in a same
geographical area which utilize multiple technology access
types.
One of the many advantages of the present invention is that it also
applies to CDMA systems with processing gain that is so low that
the typical reuse factor 1 cannot be used. For this case the
cumbersome channel planning needed for the different cells is made
automatic.
BRIEF SUMMARY OF THE INVENTION
A first telecommunications system has at least partially
geographically overlapping coverage with a second
telecommunications system, with radio transmission access in the
first telecommunications system utilizing code division multiple
access (CDMA) with a time division multiple access (TDMA)
component. A base station of the first telecommunications system
communicates over an air interface with a subscriber station and
broadcasts a common broadcast physical channel on at least an
active carrier/timeslot combination. The common broadcast physical
channel includes broadcast system identity information. The
subscriber station uses the broadcast system identity information
to distinguish between transmissions of the first
telecommunications system and the second telecommunications system.
Moreover, the subscriber station uses the broadcast system identity
information for framing transmissions in the first
telecommunications system.
The base station of the first system broadcasts at least two common
broadcast physical channels active on different timeslots.
Preferably, a fixed direct sequence CDMA code is employed for the
common broadcast physical channel.
In accordance with an instant dynamic channel selection procedure
of the present invention, the subscriber station selects a
carrier/timeslot for sending an access request to the first system
base station. The carrier/timeslot selected by the first system
subscriber station for sending an access request to the first
system base station is also used for a traffic channel. Preferably,
the first system subscriber station selects a least interfered
carrier/timeslot as the carrier/timeslot for sending an access
request to the first system base station. In response to the access
request, the first system base station sends an answer on a next
down-link carrier/timeslot included in a duplex pair with the
carrier/time slot which the first system subscriber station
selected for sending the access request to the first system base
station.
At an given moment a connection between the first system base
station and the subscriber station is borne on a physical channel
of the first telecommunications system, the physical channel being
defined by a code, timeslot, and frequency.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other objects, features, and advantages of the
invention will be apparent from the following more particular
description of preferred embodiments as illustrated in the
accompanying drawings in which reference characters refer to the
same parts throughout the various views. The drawings are not
necessarily to scale, emphasis instead being placed upon
illustrating the principles of the invention.
FIG. 1 is a schematic view of a mobile telecommunications
system.
FIG. 2 is a schematic view of plural mobile telecommunications
systems having at least partially overlapping geographical
coverage.
FIG. 3 is a diagrammatic view illustrating plural mobile
telecommunications systems operating in accordance with an
embodiment of the present invention, the plural mobile
telecommunications systems operating on plural carriers, each
plural carrier having plural carrier/timeslot channels.
FIG. 3A is a diagrammatic view of carrier/timeslot channel for
illustrating a DS-CDMA component.
FIG. 4 is a diagrammatic view illustrating channels utilized in
accordance with the present invention.
FIG. 4A is a diagrammatic view of an example of bit structure for
physical channels BPCH, DPCH, RAPCH, and FAPCH.
FIG. 4B is a diagrammatic view showing the mapping of information
type on 64 data bits of each BPCH frame.
FIG. 5 is a schematic view of a general scheme of radio interface
architecture having three protocol layers.
FIG. 6 is a diagrammatic view of an example multi-frame format of a
common broadcast physical channel (BPCH) utilized in accordance
with the present invention.
FIG. 7A and FIG. 7B are diagrammatic views respectively
illustrating a multiple switching point configuration for a
duplexed carrier and a single switching point configuration for a
duplexed carrier.
FIG. 8A and FIG. 8B are diagrammatic views respectively
illustrating (A) employment of active downlink dedicated physical
channels (DPCH) for carrying the common broadcast physical channel
(BPCH); and (B) creation of dummy common broadcast physical
channels (BPCH) when there are no active downlink dedicated
physical channels (DPCH).
FIG. 9 is a schematic view of a subscriber station according to an
embodiment of the present invention.
FIG. 10 is a flowchart showing general steps involved in a
subscriber station channel discrimination scheme according to the
present invention.
FIG. 11 is a flowchart showing general steps involved in a
subscriber station channel selection scheme according to the
present invention.
FIG. 12 is a schematic view showing an example of how blind spots
are avoided when making a seamless handover.
DETAILED DESCRIPTION OF THE DRAWINGS
In the following description, for purposes of explanation and not
limitation, specific details are set forth such as particular
architectures, interfaces, techniques, etc. in order to provide a
thorough understanding of the present invention. However, it will
be apparent to those skilled in the art that the present invention
may be practiced in other embodiments that depart from these
specific details. In other instances, detailed descriptions of well
known devices, circuits, and methods are omitted so as not to
obscure the description of the present invention with unnecessary
detail.
FIG. 1 shows a mobile telecommunications system 20.sub.1 which
comprises plural base stations BS.sub.1,1 through BS.sub.1,5
serving respective cells C.sub.1,1 through C.sub.1,5. Each base
station BS has one or more antennae which communicate over an air
interface, e.g., over radio channels, with one or more subscriber
stations located in the respective cells C. Accordingly, each base
station BS has at least one radio frequency transmitter and at
least one radio frequency receiver.
The base stations BS.sub.1,1 through BS.sub.1,5 of mobile
telecommunications system 20.sub.1 are connected by landlines or,
e.g., microwave links to a radio network controller (RNC) 22.sub.1,
also known as a base station controller. The radio network
controller (RNC) 22.sub.1 performs various functions known to those
skilled in the art, including (for example) handover operations as
required when a mobile subscriber station moves from one cell to
another. While the example mobile telecommunications system
20.sub.1 of FIG. 1 is illustrated with five cells C all controlled
by radio network controller (RNC) 22.sub.1, it will become apparent
that a mobile telecommunications system of the present invention
can be operated with fewer or more cells. Further, one or more
radio network controllers (RNCs) may be included in a mobile
telecommunications system of the present invention, with there
being no particular constraint on the number of base stations
controlled by any radio network controller (RNC), nor is it
necessary that all radio network controllers (RNCs) control the
same number of base stations.
The radio network controller (RNC) 22.sub.1 is connected by e.g.,
terrestrial lines to a mobile switching center (MSC) 24.sub.1.
Through the mobile switching center (MSC) 24 the mobile
telecommunications system 20.sub.1 connects to other mobile
telecommunications systems and to fixed or wire telecommunications
systems. Examples describing the structure and operation of the
base station, radio network controller (RNC), and mobile switching
center (MSC) nodes of a mobile telecommunications system generally
are provided in the following United States patent applications,
all of which are incorporated herein by reference: U.S. patent
application Ser. No. 09/188,102 for "Asynchronous Transfer Mode
System"; U.S. patent application Ser. No. 09/035,821 for
"Telecommunications Inter-Exchange Measurement Transfer"; U.S.
patent application Ser. No. 09/035,788 for "Telecommunications
Inter-Exchange Congestion Control"; and U.S. patent application
Ser. No. 09/071,886 for "Inter-Exchange Paging".
The present invention particularly concerns a situation in which
plural mobile telecommunications systems are operating in
uncoordinated manner in geographical areas which are at least
partially overlapping. In this regard, FIG. 2 shows the base
stations BS.sub.1,1 through BS.sub.1,5 and respective cells
C.sub.1,1 through C.sub.1,5 (illustrated in solid lines) of mobile
telecommunications system 20.sub.1 along with base stations and
cells of mobile telecommunications system 20.sub.2 and mobile
telecommunications system 20.sub.3. The mobile telecommunications
system 20.sub.2 comprises base stations BS.sub.2,1 through
BS.sub.2,5 and respective cells C.sub.2,1 through C.sub.2,5
(illustrated in dotted lines in FIG. 2); mobile telecommunications
system 20.sub.3 comprises base stations BS.sub.3,1 through BS3,5
and respective cells C.sub.3,1 through C.sub.3,5 (illustrated in
dotted-dashed lines in FIG. 2). Thus, it can be ascertained from
the subscript scheme that the first element of a subscript for a
base station or cell identifies its mobile telecommunications
system, while the second subscript for a base station or cell
identifies the particular base station or cell within that
system.
To at least some extent there are regions of geographical overlap
in the coverage areas served by the respective mobile
telecommunications systems 20.sub.1, 20.sub.2, and 20.sub.3. For
example, subscriber stations SS.sub.1a and SS.sub.1b belong to
mobile telecommunications system 20.sub.1 and are situated as shown
in FIG. 2. Subscriber station SS.sub.1a is within geographical
range of all three mobile telecommunications systems. In this
regard, it can be seen in FIG. 2 that subscriber station SS.sub.1a
is situated in cell C.sub.1,1 of mobile telecommunications system
20.sub.1 in cell C.sub.2,3 of mobile telecommunications system
20.sub.2 ; and in cell C.sub.3,2 of mobile telecommunications
system 20.sub.3. Subscriber station SS.sub.1b is at the range limit
of base station BS.sub.1,3 and operates at maximum power, which
would block BS.sub.3,3 if used simultaneously on the same carrier.
The present invention provides a way for subscriber station
SS.sub.1a to discriminate between the transmissions of the three
mobile telecommunications systems to which it is subjected, for
ascertaining what transmissions are intended for subscriber station
SS.sub.1a, and for dynamically selecting channels for use in radio
communications involving subscriber station SS.sub.1a. Hereinafter,
for sake of simplicity, all reference to subscriber station SS
should be understood to be the example subscriber station
SS.sub.1a.
As mentioned above, the invention also addresses the problem for a
single system when channel reuse 1 is not possible between the
cells of the system. For this case proper carrier/time-slot
channels (see below) are automatically selected so that the
connections in adjacent cells do not interfere with each other.
Modern CDMA systems have the capability to trade processing gain
against instant user bit rate. Thus at some limit when increasing
the used bit rate, reuse 1 is no longer possible, which would
result in a sudden cumbersome "manual" channel planning for the
different cells. The invention also avoids this need for
planning.
Each of the mobile telecommunications systems shown in FIG. 2 have
multiple radio frequency carriers and utilize code division
multiple access (CDMA) with a time division multiple access (TDMA)
component, as illustrated in FIG. 3. For example, each mobile
telecommunications system uses the same four radio frequency
carriers F.sub.1 -F.sub.4. Each radio frequency carrier F is
divided into plural timeslots. In the particular example shown in
FIG. 3, each radio frequency carrier F is divided into sixteen (16)
timeslots. Each combination of carrier and timeslot is herein
referred to as a carrier/timeslot channel and is abbreviated both
as "C/TS" and "CH". For example, the mobile telecommunications
system 20.sub.1 has carrier/timeslot channels CH.sub.1,1,1 through
CH.sub.1,1,6 for radio frequency carrier F.sub.1, while for radio
frequency carrier F.sub.2 the mobile telecommunications system
20.sub.1 has carrier/timeslot channels CH.sub.1,2,1 through
CH.sub.1,2,16. Similarly, for a given radio frequency carrier
F.sub.x the mobile telecommunications system 20.sub.2 has
carrier/timeslot channels CH.sub.2,x,1 through CH.sub.2,x,16, while
mobile telecommunications system 20.sub.3 has carrier/timeslot
channels CH.sub.3,x,1 through CH.sub.3,x,16. Thus, it can be seen
that in the subscript notation for a carrier/timeslot channel the
first element refers to the number of the mobile telecommunications
system (e.g. mobile telecommunications system 20.sub.1, 20.sub.2,
or 20.sub.3); the second element refers to the radio frequency
carrier (e.g., F.sub.1, F.sub.2, F.sub.3, or F.sub.4); and the
third element refers to the timeslot (e.g., one of the timeslots 1
through 16).
Examples of duplex carrier/timeslot channels are provided in FIG.
7A and FIG. 7B. As used herein, the term "duplex" means a pairing
of an uplink carrier/timeslot channel and a downlink
carrier/timeslot channel, within the same frame and on the same
carrier. The frames of FIG. 7A and FIG. 7B each are of 10
milliseconds in time duration, and each have sixteen slots (each
slot being 625 microseconds). FIG. 7A shows a multiple switching
point configuration wherein a downlink carrier/timeslot channel is
immediately followed by its paired uplink carrier/timeslot channel.
As an example, the shaded downlink carrier/timeslot channel
CH.sub.1,1,3 is immediately followed by the shaded uplink
carrier/timeslot channel CH.sub.1,1,4 which is paired with downlink
carrier/timeslot channel CH.sub.1,1,3. FIG. 7B, on the other hand,
shows a single switching point configuration for a duplex wherein
the first eight carrier/timeslot channels are for the downlink and
the last eight carrier/timeslot channels of the frame are for the
uplink. In FIG. 7B, for example, the shaded downlink
carrier/timeslot channel CH.sub.1,1,2 is paired with the shaded
uplink carrier/timeslot channel CH.sub.1,1,10.
Returning to FIG. 3, the FDMA access component of the present
invention thus is attributable to the employment of multiple radio
frequencies. The TDMA access component is attributable to the
timeslotting of each radio frequency carrier. The CDMA access
component of the present invention is attributable to the fact that
each carrier/timeslot channel accommodates plural physical channels
PCH. As used herein, a physical channel PCH is an association of
one code, one timeslot, and one frequency. The physical channels
(PCH) of each carrier/timeslot channel (C/TS) are discriminated on
the basis of direct sequence (DS) CDMA spreading codes. Such is
illustrated in FIG. 3A, which shows a representative one of the
carrier/timeslot channels CH.sub.1,1,6 has having physical channels
PCH.sub.1,1,6,1 through PCH.sub.1,1,6,n. The first three elements
of the subscript of a physical channel PCH thus corresponds to the
first three elements of the subscript of its carrier/timeslot
channel. The fourth element of the subscript of a physical channel
PCH refers identifies a particular DS-CDMA spreading code
utilizable for a telecommunications connection (e.g., for a call).
Thus, on each carrier/timeslot channel, there can be up to n number
of physical channels PCH, each separated by a DS-CMDA code. The
transmission direction for all physical channels PCH on the same
carrier/timeslot channel on the same carrier is the same. A base
station equipped with one radio transceiver is only able to operate
on one time slot on one carrier at the same time. For sake of
simplicity the physical channels PCH of the other carrier/timeslot
channels are not shown in FIG. 3, but it should be appreciated that
each carrier/timeslot channel CH has multiple physical channels PCH
in the same manner as shown in FIG. 3B for carrier/timeslot channel
CH.sub.1,1,6.
The subscriber station SS, whether idle or in communication, is
always locked to a closest (strongest) base station of its own
system, as in usual DC-CDMA practice. Automatic handover is made as
soon as another base station of the same system becomes stronger.
Being locked to the strongest base station is important for
efficient access channel reuse and link robustness, which leads to
high capacity. The handover should be "seamless", which means "make
before break", as is important for high quality of service.
FIG. 5 shows a general scheme of radio interface architecture
having three protocol layers: L1 (Physical Layer); L2 (Data Link
Layer); and L3 (Network Layer). The Data Link Layer (L2) includes
Radio Link Control (RLC) and Medium Access Control (MAC). The
Network Layer (L3) is divided into a control plane (C-plane) and
user plane (U-plane). Layer 1 is very different for TDD and FDD,
for which reason the present invention makes Layer 2 and Layer 3
the same for TDD and FDD as much as possible. Physical layer (PHL)
services to the Medium Access Control (MAC) are identified in terms
of transport channels. Some of the transport channels are common
channels which are used by plural mobile subscriber stations, and
include a random access channel (RACH); a forward access channel
(FACH); a broadcast channel (BCH); and a paging channel (PaCH).
Others of the transport channels are dedicated channels (DCH),
e.g., channels which are dedicated to a particular connection for
user data and control information between a base station and a
mobile subscriber station.
The present invention employs four types of physical channels--the
common broadcast physical channel (BPCH); the dedicated physical
channel (DPCH); the random access physical channel (RAPCH); and the
forward access physical channel (FAPCH). FIG. 4 indicates, for each
physical channel type: (1) the one or more transport channels
included in the physical channel, (2) the direction (e.g.,
downlink, uplink) of the physical channel, and (3) the type of
spreading employed on the physical channel.
As shown in FIG. 4, the dedicated physical channel (DPCH) includes
the dedicated transport channel (DCH) and occurs both on the uplink
and the downlink. Both fixed and variable spreading are allowed on
the dedicated physical channel (DPCH).
The random access physical channel (RAPCH) is used when a
subscriber station desires to obtain a radio access, e.g., to make
a telecommunications connection such as to make a call. The random
access physical channel (RAPCH) is thus in the uplink direction,
and includes the RACH transport channel. In the illustrated
embodiment, the random access physical channel (RAPCH) has fixed
spreading codes. As explained further below, in the present
invention the random access physical channel (RAPCH) is available
on every potential uplink carrier/timeslot channel. Moreover, the
subscriber station SS dynamically selects the carrier/timeslot
channel to use for the random access physical channel (RAPCH), and
after acceptance by the base station that same carrier/timeslot
channel is employed as the dedicated physical channel (DPCH) for
the connection.
The forward access physical channel (FAPCH) is a downlink direction
channel used, for example, by a base station in responding to a
random access request or a request for packet communications. The
forward access physical channel (FAPCH) thus includes the forward
access transport channel (FACH). In the illustrated embodiment, the
forward access physical channel (FAPCH) has fixed spreading
codes.
As also shown in FIG. 4, the common broadcast physical channel
(BPCH) includes the broadcast (BCH) and paging (PaCH) common
transport channels. Thus, the common broadcast physical channel
(BPCH) carries broadcast system identities and capabilities, as
well as downlink synchronization information to the subscriber
station and in addition the downlink paging channel (PaCH) to the
subscriber station. In the illustrated embodiment, the common
broadcast physical channel (BPCH) has fixed spreading codes.
FIG. 4A shows an example of how to structure the physical channels
common broadcast physical channel (BPCH), random access physical
channel (RAPCH), forward access physical channel (FAPCH), and
dedicated physical channel (DPCH). A time slot comprises a midamble
of 512 chips and 1024 data chips on each side. There is also a
guard space GS of 96 chips before the next slot begins. The
midamble is used to derive slot and bit synchronization, not
necessarily any system identification. A preamble at the beginning
of the slot could, of course, have been used instead of a midamble.
The midamble could be common for all systems, but does not
necessarily need to be so. In other embodiments, the midamble could
be geographically specific for each base station in a system, or be
specific for each simultaneous PCH on a C/TS. This last version
could be useful if a specific decoding technique called joint
detection is implemeted. Thus, the specific spreading codes of FIG.
4 that differentiate the different physical channels (PCH) apply
only to the remaining 1024+1024=2048 chips, which thus will carry
the information content of the physical channels. With a variable
spreading factor (SF) from 1 to 32, there are between 64 and 2048
bits available on each physical channel to carry information.
Supposing the frame rate to be 100 Hz (10 ms frame length), the raw
bit rate will be 6.4 kbps with SF=32 and 204.8 kbps with SF=1. If
there are 16 timeslots, the total chip rate will be
16.times.(512+2048+96).times.100=4,249,600 chips/sec. This could be
implemented on 5 MHz wide frequency carriers depending on type of
modulation.
In a preferred embodiment, SF=16 for RAPCH, FAPCH and DPCH. This
will provide 12.8 kbps (128 bits per slot). This is suitable for a
DPCH speech service and also for quick RACH and FACH transfer. How
to compose a RACH or FACH message is common knowledge and does not
need further description here. An example is provided for the
mapping of the BCH and PaCH information on BPCH.
In a preferred embodiment, SF=32 or higher for BPCH, because it is
very important that the BPCH information be received by all idle
subscriber stations SS. FS=32 will provide 6.4 kbps or 64 bits per
slot. All BPCH information cannot fit in one slot, but that is not
needed. Therefore, a multi-frame has been defined in FIG. 6
comprising 16 frames, each frame comprising 16 carrier/timeslot
channels (see FIG. 7B) CH.sub.x,x,1 to CH.sub.x,x,16. In FIG. 6,
the 16 carrier/timeslot channels are shown only for the first
frame, it being understood that all 16 frames also have 16
carrier/timeslot channels. It is possible to fit all the different
types of BPCH information in different frames in two or more
multiframes. To do this and to be able to receive the relevant type
of BPCH information, even if it is not known exactly in which frame
it comes, the 64 data bits in one frame are divided into 4 bits to
describe the information type and the remaining 60 bits for coding
the wanted information (as shown in FIG. 4B).
The type of information required in BCH and PaCH for mobile systems
is common knowledge, but Table 1 gives examples of required
information types and how they could be identified and mapped on
different frames in a multi-frame. The different information types
in Table 1 are mapped on the multi-frame with different priorities.
Paging has the highest priority and can be sent on all odd frames.
A subscriber station in an idle locked current saving mode only has
to listen to frame 1. All paging frames will have a bit that tells
if the next odd frame within a multi-frame has any paging content
or not. Thus, Frame 1 will inform if paging information is present
in any subsequent odd frame in the present multi-frame. If there is
no paging information to be sent in an odd frame, the subscriber
station can go back to sleep until frame occurs again.
TABLE 1 Examples of Identification and Mapping of Information Types
On Different Frames In A Multiframe Information Content in
information To be Type field (60 bits transmitted Type of Code
including error in frame information (4 bits) detection bits)
number PaCH, Paging 0000 Global or temporary 1, 3, 5, 7, 9, mobile
user identity 11, 1, 3, 15 BCH part 1 0001 System identity and 2
System transmitting base identity station identity BCH part 2 0010
Slot number in the 4 Synchronization frame and frame number in the
multi-frame for the received slot (Note that chip, bit and slot
synchronization is provided via the midamble) BCH part 3 0011 Coded
information 6 System capabilities on system capabilities Reserved
for other 0100-1111 Any other type of 8, 10, 12, types of
information information 14, 16
Some CDMA systems define a specific synchronization channel, which
could or could not include some system identify information. Such
is not required in this invention, because the chip, bit, and slot
synchronization is provided in the common structure with the
midamble of the common broadcast physical channel (BPCH) (and all
other PCHs) [see FIG. 4A], and remaining synchronization and system
identity information is provided by the BCH (see Table 1).
The common broadcast physical channel (BPCH) is distributed on each
active downlink carrier/timeslot channel CH, e.g., those
carrier/timeslot channels with an active dedicated physical channel
(DPCH). FIG. 8A shows a single switching point configuration for a
duplex channel for radio frequency carrier F.sub.1 in mobile
telecommunications system 20.sub.1 wherein each of carrier/timeslot
channels CH.sub.1,1,1, CH.sub.1,1,4, CH.sub.1,1,5, and CH.sub.1,1,8
have active traffic (as indicated by shading). Therefore each of
carrier/timeslot channels CH.sub.1,1,1, CH.sub.1,1,4, CH.sub.1,1,5,
and C.sub.1,1,8 carries the common broadcast physical channel
(BPCH).
If there are less than two active downlink dedicated physical
channels (DPCH), the base station creates one or two dummy common
broadcast physical channels (BPCH) on different carrier/timeslot
channels as needed. A BPCH is called a dummy BPCH when the base
station has no other physical channel on the specific
carrier/timeslot channel used by the BPCH. In the situation shown
in FIG. 8B, there are no active downlink dedicated physical
channels (DPCH) emanating from a base station, which means that the
base station has to create two dummy common broadcast physical
channels (BPCH) on different carrier/timeslot channels such as
CH.sub.1,1,4 and CH.sub.1,1,8. The two dummy common broadcast
physical channels (BPCH) such as CH.sub.1,1,4 and CH.sub.1,1,8 thus
each carry the common broadcast physical channel (BPCH). If there
were only one active downlink dedicated physical channel (DPCH)
emanating from a base station, that base station would have to
create one dummy common broadcast physical channel (BPCH). A
subscriber station equipped with one radio transceiver is only able
to operate on one time slot on one carrier at the same time (but
could in a multi-carrier case operate on different carriers on
different time slots). By always having BPCH active on at least two
different time slots, a subscriber station may be in communication
on one time slot, and still be able to detect surrounding cells of
the same system and make a quick handover to them as soon as they
become stronger than the own base station (due to the mobility of
the subscriber station).
The dummy common broadcast physical channels (BPCH) are set up and
kept on a least interfered carrier/timeslot channel by the base
station. In order to do so, the base station periodically (e.g.,
every second) checks the interfering power for each
carrier/timeslot channel and moves the dummy common broadcast
physical channel (BPCH) if required to the new best
carrier/timeslot channels. In so doing, a short pause may be made
during a frame when idle locked handsets are sleeping. Since the
common broadcast physical channel (BPCH) is activated on every
downlink carrier/timeslot channel with traffic (e.g, DPCH), the
dummy common broadcast physical channels (BPCH) are released when
two carrier/timeslot channels are carrying traffic. When needed,
the dummy common broadcast physical channels (BPCH) are activated
again, so that always at least two downlink carrier/timeslot
channels on the same carrier have dummy common broadcast physical
channels (BPCH). The base station may select the differing
timeslots to have at least one slot separation therebetween in
order, e.g., to avoid blind slots at seamless handover. FIG. 12
provides an example of how blind slots are avoided when making a
seamless handover from cell 1 to cell 2. If there had been no
traffic in cell 2, then the traffic bearer on carrier 2 would have
been a second dummy BPCH.
Thus, in accordance with the present invention, at least two of the
carrier/timeslot channels from each base station have the common
broadcast physical channel (BPCH). All carrier/timeslot channels
which have active downlink dedicated physical channels (DPCH) also
carry the common broadcast physical channel (BPCH) as in the manner
illustrated in FIG. 8A.
FIG. 9 shows components of an example subscriber station SS. The
subscriber station SS described herein can be any type of mobile
telecommunications unit, including (for example) a mobile telephone
or portable computer. The subscriber station SS has its antenna
9-10 suitably connected to receiver & demodulator 9-12 and to
transmitter & modulator 9-14 by switch 9-13. Overall
operational control of subscriber station SS, and the application
logic for subscriber station SS, resides at processor and
control/application logic 9-15. Hereafter, for simplicity,
processor and control/application logic 9-15 is referred to simply
as processor 9-15.
On a receive side of subscriber station SS, information demodulated
by receiver & demodulator 9-12 is applied to framer and slot
synchronization unit 9-16 and DS-code removal unit 9-18. Both
receiver & demodulator 9-12 and framer and slot synchronization
unit 9-16 are controlled by processor 9-15, as shown by control
lines emanating from processor 9-15. The subscriber station SS
operates using one or more spreading codes, including a
predetermined or native direct sequence (DS) spreading code and (in
the illustrated embodiment) a fixed spreading code which is
employed for certain physical channels as described above with
reference to FIG. 4. The spreading codes employed by subscriber
station SS are stored at processor 9-15. The processor 9-15
provides DS-code removal unit 9-18 with the parameters of the
spreading codes over line 9-20. Using the code parameters supplied
by processor 9-15, the DS-code removal unit 9-18 determines whether
the incoming transmission has the same spreading code as is native
to subscriber station SS or the fixed spreading code used for
common physical channels of the mobile telecommunications system
20.sub.1 to which the subscriber station SS belongs. If the DS-code
removal unit 9-18 detects the native or fixed spreading code, it
decodes the incoming transmission. The decoded data obtained from
the incoming transmission is applied to a multiplexer and
demultiplexer unit (mux & demux 9-22). Data is bidirectionally
communicated between mux & demux 9-22 and processor 9-15 on
data bus 9-24; control information is bidirectionally communicated
between mux & demux 9-22 and processor 9-15 on control bus
9-26.
The control and applications logic included in processor 9-15 of
subscriber station SS can be of various types. Examples of the
content of a typical mobile control and application logic executed
by processor 9-15 are provided in the further description below.
Moreover, the processor 9-15 communicates with a user interface
9-30 over interface bus 9-32. For a typical mobile telephone, for
example, the user interface 9-30 can include the telephone
keypad.
On its transmit side, data gated from processor 9-15 through mux
& demux 9-22 is applied to DS-code add unit 9-40. The DS-code
add unit 9-40 encodes the data using the native or fixed DS code
parameters received from processor 9-15 over line 9-42. The encoded
data is then modulated by transmitter & modulator 9-14, and put
in the correct frame ad carrier/timeslot channel by framer and slot
synchronization unit 9-16. The framed information is then
transmitted to antenna 9-10 via transmitter & modulator 9-14.
The transmitter & modulator 9-14 is connected to processor 9-15
for, e.g., control purposes. The switch 9-13 is controlled from
framer and slot synchronization unit 9-16 and connects the antenna
9-10 to the transmitter 9-14 during transmit slots and to the
receiver 9-12 during receive slots.
With the physical channels and subscriber station SS having been
described, it can now be appreciated (e.g., with reference to the
general steps of FIG. 10) how the subscriber station SS situated as
shown in FIG. 2 communicates with its mobile telecommunications
system 20.sub.1 (amidst the extraneous transmissions of mobile
telecommunications system 20.sub.2 and mobile telecommunications
system 20.sub.3). For example, if it is assumed subscriber station
SS is powered on as shown in step 10-1 of FIG. 10. After start up,
the receiver & demodulator 9-12 of subscriber station SS
monitors its native frequency (or frequencies), i.e., a
predetermined one or more of the radio frequency carriers F.sub.1
-F.sub.4, as shown by Step 10-1. Step 10-2 shows that the
information received on the native frequency is decoded by DS-code
removal unit 9-18, which attempts to detect a spreading code used
for the common broadcast physical channel (BPCH) of mobile
telecommunications system 20.sub.1. In the illustrated embodiment,
the spreading code employed for common broadcast physical channel
(BPCH) is a fixed spreading code, which means that the spreading
code for common broadcast physical channel (BPCH) is used by all
(or at least plural) subscriber stations for detecting common
broadcast physical channel (BPCH). When the DS-code removal unit
9-18 detects the spreading code employed for the common broadcast
physical channel (BPCH) of its mobile telecommunications system
20.sub.1, the mux & demux 9-22 sends the BPCH information to
processor 9-15 which examines the information content of the common
broadcast physical channel (BPCH) at step 10-3. In particular, at
step 10-3 the system identifier (see system identity in Table 1)
included in the common broadcast physical channel (BPCH) is
examined. Then, at step 10-4 processor 9-15 checks whether the
system identifier carried in the common broadcast physical channel
(BPCH) is the same as the system identifier prestored in the
application logic of processor 9-15 of subscriber station SS. In
this regard, it should be kept in mind that other mobile
telecommunications systems could use the same spreading code for
their common broadcast physical channel (BPCH), so the checks of
step 10-3 and step 10-4 are necessary.
In an alternative embodiment, the DS-code removal unit 9-18 is
loaded from processor 9-15 with the system identifier, and the
steps 10-3 and 10-4 are hardware coded in DS-code removal unit
9-18. This is useful if the processor 9-15 is not fast enough to
quickly perform the system identification.
If it is determined at the decoded common broadcast physical
channel (BPCH) is, in fact, for the mobile telecommunications
system 20.sub.1 to which subscriber station SS subscribes, at step
10-5 the subscriber station SS obtains the frame sequence number,
carrier/timeslot channel number, and synchronization information
from the common broadcast physical channel (BPCH) (see Table 1) and
stores it in the processor 9-15. With the frame sequence number,
carrier/timeslot channel number, and synchronization information
thusly gleaned from the common broadcast physical channel (BPCH),
the subscriber station SS can lock to the framing of its mobile
telecommunications system 20.sub.1 and thereby discriminate between
transmissions of mobile telecommunications system 20.sub.1 and the
transmissions of other mobile telecommunications systems.
The foregoing description of the steps of FIG. 10 concerns the
monitoring of a native frequency for detecting the common broadcast
physical channel (BPCH), and thereby locking to the mobile
telecommunications system to which subscriber station SS belongs.
The present invention also provides a dynamic channel selection
process as described below in conjunction with the basic steps
illustrated in FIG. 11.
Step 11-0 of FIG. 1 shows processor 9-15 of subscriber station SS
receiving a user request for radio access from user interface 9-30.
Such user request for access can take the form of the user dialing
a called party number and activating a send key on user interface
9-30. The user request may also be activation of the send key as
response to a ring signal from the subscriber station being paged.
At this juncture, the subscriber station SS is synchronized to its
mobile telecommunications system 20.sub.1 in accordance with the
procedure above described in connection with FIG. 10. Such being
the case, at step 11-1 the processor 9-15 of subscriber station SS
monitors each time slot of the frame. If it is determined at step
11-2 that the frame has a timeslot which carries a dummy common
broadcast physical channel (BPCH) [see FIG. 8B], then at step 11-33
the subscriber station SS determines whether the timeslot selected
at step 11-2 has a signal to interference ratio (SIR) which exceeds
a predetermined minimum threshold T.sub.SS required by the
subscriber station SS. The signal to interference ratio (SIR) is
measured in receiver & demodulator 9-12. If the minimum
threshold T.sub.SS is not exceeded, execution returns to step 11-2
to ascertain whether another carrier/timeslot channel carries a
dummy common broadcast physical channel (BPCH), and if so the check
of step 11-3 is repeated. Should it be determined at step 11-3 that
the minimum threshold T.sub.SS is exceeded by a carrier/timeslot
channel which carries a dummy common broadcast physical channel
(BPCH), then at step 11-4 the subscriber station SS steps up a
radio access request to the base station. The radio access request
is carried over a random access physical channel (RAPCH) which is
in the duplex pair with the carrier/timeslot channel which carried
the dummy common broadcast physical channel (BPCH). For example, if
the subscriber station SS selected the carrier/timeslot channel
CH.sub.1,1,4 in FIG. 8B that carried a dummy common broadcast
physical channel (BPCH), the access request of step 11-4 is sent on
a random access physical channel (RAPCH) carried in
carrier/timeslot channel CH.sub.1,1,12 (see FIG. 8B).
After the access request of step 11-4 is sent, at step 11-35 the
processor 9-15 of subscriber station SS checks whether an
affirmative response to the access request is received in a forward
access physical channel (FAPCH) in the first available downlink
carrier/timeslot channel of the duplex channel pair. In the
foregoing illustration involving FIG. 8B, wherein the access
request occurred of step 11-4 occurred in carrier/timeslot channel
CH.sub.1,1,12, the forward access physical channel (FAPCH) is
expected at step 11-5 to be in carrier/timeslot channel
CH.sub.1,1,4, of the next frame. If an affirmative response from
the base station to the access request is provided in the
appropriate carrier/timeslot channel as determined at step 11-5, at
step 11-6 the subscriber station SS sets up a duplex
carrier/timeslot channel utilization.
In the duplex utilization set up at step 11-6, the uplink
carrier/timeslot channel which carried the access request (e.g.,
carrier/timeslot channel CH.sub.1,1,12 of FIG. 8B) is employed for
the uplink dedicated physical channel (DPCH) for the connection,
while the downlink carrier/timeslot channel which still carries the
dummy common broadcast physical channel (BPCH) and carried the
forward access physical channel (FAPCH) is employed as the downlink
dedicated physical channel (DPCH) for the connection. In the
illustrated embodiment, the messages sent prior to step 11-6 (i.e.,
prior to establishing the duplex carrier/timeslot channel
utilization) are encoded with fixed spreading codes, but thereafter
the spreading code native to the subscriber station SS is utilized
so that the connection acquires one of the physical channels PCH as
illustrated in FIG. 3. For example, on the uplink the connection
may utilize physical channel PCH.sub.1,1,12,1 (e.g., a physical
channel PCH using the first spreading code of carrier/timeslot
channel CH.sub.1,1,12), and on the downlink the connection may
utilize physical channel PCH.sub.1,1,4,1 (e.g., a physical channel
PCH using the first spreading code of carrier/timeslot channel
CH.sub.1,1,4).
In connection with step 11-5, before the base station sends an
affirmative response to the access request of step 11-4, the base
station must first determine whether the signal to interference
ratio (SIR) as measured at the base station for the uplink
carrier/timeslot channel involved in the request (e.g.,
carrier/timeslot channel CH.sub.1,2,12) exceeds a minimum threshold
T.sub.BS. If the minimum threshold T.sub.BS is exceeded, the base
station can send an affirmative response on the forward access
physical channel (FAPCH) to the subscriber station SS. If the
minimum threshold T.sub.BS is not exceeded, the base station does
not respond to the access request. When the response is not
received by subscriber station SS at step 11-5, the subscriber
station SS returns to step 11-2 to hunt for another dummy common
broadcast physical channel (BPCH).
If no dummy common broadcast physical channels (BPCH) exist, or if
the carrier/timeslot channels of the existing dummy common
broadcast physical channels (BPCH) are unusable, at step 11-10 the
subscriber station SS detennines whether there are any active
dedicated physical channels (DPCH), from the base station currently
handling the subscriber station SS where the SIR on the related
carrier/timeslot channel exceeds the minimum threshold T.sub.SS of
subscriber station SS. If such carrier/timeslot channel exists, at
step 11-1 the processor 9-15 of subscriber station SS selects the
carrier/timeslot channel with the largest SIR. Thereafter, the
subscriber station SS sends its access request (at step 11-12) and
checks for a response (at step 11-13) prior to setting up a new
duplex DPCH using carrier/timeslot channel with the largest SIR
(step 11-14). It will be appreciated that step 11-12, step 11-13,
and step 11-14 are essentially identical to steps 11-4, 11-5, and
11-6, respectively, discussed above, but involve the
carrier/timeslot channel having a dedicated physical channel (DPCH)
with the largest SIR rather than a carrier/timeslot channel with
dummy common broadcast physical is channel (BPCH).
If no answer to the access request is received by subscriber
station SS from the base station at step 11-13, the subscriber
station SS returns to step 11-10 to determine if there are any
other carrier/timeslot channels with dedicated physical channels
(DPCH) at its own base station which exceed the minimum threshold
T.sub.SS of subscriber station SS. If such other carrier/timeslot
channels exist, step 11-12, step 11-13, and step 11-14 are
performed relative to the carrier/timeslot channel with dedicated
physical channel (DPCH) with the next largest SIR.
If there are no qualifying carrier/timeslot channels with dedicated
physical channels (DPCH) for the base station of the subscriber
station SS as determined at step 11-10, at step 11-20 the
subscriber station SS determines whether any carrier/timeslot
channel with detectable dedicated physical channels (DPCH) from a
neighboring base station exceed the minimum threshold T.sub.SS of
subscriber station SS. If one or more such dedicated physical
channels (DPCH) exist from the neighboring base station, step 11-21
through 11-24 are performed relative to the carrier/timeslot
channel of such dedicated physical channel (DPCH). Steps 11-21
through 11-24 are essentially identical to step 11-12, step 11-13,
and step 11-14, it being understood that steps 11-21 through 11-24
pertain to a carrier/timeslot channel with dedicated physical
channel (DPCH) from a neighboring base station instead of the
subscriber station SS's own base station (the base station to which
the subscriber station is synchronized and locked).
If there are no qualifying carrier/timeslot channels with dedicated
physical channels (DPCH) for a neighboring own system base station
of the subscriber station SS as determined at step 11-10, at step
11-30 the subscriber station SS sets up a new duplex DPCH
connection using a least interfered carrier/timeslot channel on a
randomly selected carrier (F.sub.1 -F.sub.4), if more than one
carrier is supported by the base station to which the subscriber
station SS is locked. The setting up of a new duplex DPCH
connection is understood from comparable steps 22-21 through 11-24
described above.
The minimum threshold T.sub.SS at subscriber station SS and the
minimum threshold T.sub.BS at the base station depend on several
factors. These factors include the amount of processing gain, the
number of connections already using the carrier/timeslot channel,
and the performance of the specific method of detecting codes and
discriminating between codes on the same carrier/timeslot channel.
The optimum thresholds could be different for different services,
and might be adapted from call to call dependent on type of
call.
Thus, for call setup or handover, the subscriber station SS selects
a least interfered duplex carrier/timeslot channel and makes direct
DPCH setup (20 ms) via the random access physical channel (RAPCH)
and forward access physical channel (FAPCH) on the duplex
carrier/timeslot channel to the strongest base station. This
provides quick DPCH access. The handover is decentralized and
controlled by subscriber station SS. This avoids complex
coordination or tricky channel selection requirements on the fixed
infrastructure. The fact that the DPCH selection basically is made
by the subscriber station SS does not prevent supplementary
information like blind spot information or control from the system
side.
The present invention thus provides an instant dynamic channel
selection procedure for Direct Sequence CDMA mobile radio systems
that have a TDMA component and/or multiple carriers (e.g., FDMA).
By "instant" is meant that the channel selection is made in
accordance with the need of a base station, e.g., on a per call or
per connection basis. Advantageously, the selection is made by the
handset, i.e., the subscriber station SS.
The instant dynamic channel selection of the present invention is
based on defining the least interfered channel (LIC). For a duplex
traffic channel, the least interfered channel (LIC) is the duplex
carrier/timeslot channel pair in which the downlink as the lowest
radio signal strength indicator (RSSI) of all carrier/timeslot
channel pairs measured at the subscriber station SS. For a common
broadcast physical channel (BPCH), or downlink simplex traffic
channels, the least interfered channel (LIC) is the downlink of the
duplex carrier/timeslot channel pair where the highest RSSI of any
of the two slots of the carrier/timeslot channel pair has the
lowest value of all carrier/timeslot channel pairs as measured at
the base station. It is also possible, as an alternative for duplex
(or up-link simplex) traffic channels to define the least
interfered C/TS channel as the slots belonging to the duplex
carrier/timeslot channel pair where the highest RSSI of any of the
two slots of the carrier/timeslot channel pair has the lowest value
of all carrier/timeslot channel pairs as measured at the subscriber
station.
Efficient instant dynamic channel selection is based on the
statistical law of large numbers. This means that there shall be as
many choices as possible for selecting a good channel. A good
channel is a channel that has a high probability to have a good
enough signal to interference ratio (SIR), and also has a low
probability of interfering with an existing connection in the same
system or any other system. Efficient application of instant
dynamic channel selection implies that setting up a new call
sometimes will interfere with an already-existing call, which then
will be forced to escape (e.g., make handover) to another
carrier/timeslot channel. This is no problem so long as not only
traffic channels, but also broadcast, synchronization, paging and
access channels are dynamically selected (which occurs in the
present invention).
Thus, in the present invention there is no specific carrier or
carrier/timeslot channel for BPCH or RAPCH or FAPCH, in which case
those could be interference so that the whole base station would be
inoperable. A fixed position for these channels would force
planning, as well as costly and impractical time synchronization
between all systems.
Advantageously, the present invention makes the instant dynamic
channel selection procedure independent of the amount of processing
gain applied by the DS-CDMA coding process. More processing gain
means that there can be more calls on the same time slot. The
invention thus is pertinent for different and varying processing
gain implemented on different channels and connections. To
implement this feature down to very low processing gains (e.g.,
even down to 1), there may be a need for parts of common broadcast
physical channel (BPCH) to be time multiplexed on dedicated
physical channel (DPCH). This time multiplexing can be made within
each slot, or be made over several slots as appropriate. Where the
processing gain is low on some or all carrier/timeslot channels so
that only one physical channel is allowed, the whole common
broadcast physical channel (BPCH) has to be time multiplexed on
dedicated physical channel (DPCH) on these carrier/timeslot
channels. This permits common broadcast physical channel (BPCH) to
be available on every active carrier/timeslot channel. In such
circumstances, the procedure of FIG. 11 would be streamlined by
bypassing the checks of 11-10 and 11-20. It in this regard, the
forward access physical channel (FAPCH) would also have to be
similarly time multiplexed.
The present invention also advantageously allows coexistence of
uncoordinated installations of DS-CDMA mobile radio systems on a
common spectrum allocation. The invention is primarily intended
for, but not limited to, numerous residential and office systems
applied in the same geographical area. For example, the mobile
telecommunications systems 20.sub.1 -20.sub.3 shown in FIG. 2 can
be operated by different companies or institutions which are all
geographically located within an office park or technology zone.
The invention can also be applied on a spectrum exclusively
allocated for one public operator, or on a common (e.g.,
unlicensed) allocation available for a multitude of operators.
The present invention thus enables efficient deployment of
different geographical overlapping DS-CMDA systems by letting the
connections of the different systems make use of different
carrier/timeslot channels. The different carrier/timeslot channels
provide the necessary isolation between the radio connections of
the different systems, with the instant dynamic channel selection
procedure of the present invention being used for selecting the
proper carrier/timeslot channel.
Yet another advantage of the present invention is that can coexist
with proper classes of multi-carrier (MC) TDMA systems which use
instant dynamic channel selection.
In one particular example implementation, the four radio frequency
carriers F.sub.1 -F.sub.4 are provided in the frequency range of
1900-1920 MHz. The invention is not limited to the use of four
radio frequency carriers, nor the provision of sixteen timeslots
per radio frequency carrier. It will be appreciated that a greater
or lesser number of radio frequency carriers can be employed, as
well as a greater or lesser number of timeslots per radio frequency
carrier. In fact, the present invention can be practiced even if
only one radio frequency carrier is employed by a mobile
telecommunications system.
The embodiments illustrated herein have fixed spreading codes for
the common broadcast physical channel (BPCH), random access
physical channel (RAPCH), and forward access physical channel
(FAPCH). Fixed spreading codes are predetermined DS-CDMA codes
which can be immediately applied to these physical channels for
decoding and encoding, thereby providing some advantage. It should
be understood, however, that the present invention encompasses
embodiments in which variable spreading codes are also allowed on
these physical channels.
Whereas FIG. 9 has shown components of a subscriber station SS, for
the purposes of the present invention it should be understood that
similar components can reside in an embodiment of the base station.
In such base station embodiment, the processor controls the base
station and executes application logic appropriate for the base
station, and is connected to a base station user interface.
The embodiments illustrated herein CDMA mobile telecommunications
systems with a TDMA component. However, it should also be
understood that the TDMA component is not necessary, and that the
principles of the invention can also be applied for CDMA with only
a FDMA component. That is, while the instant dynamic channel
selection as described herein is generally applied both in the
frequency and time domains, the invention is equally applicable if
only one of those domains is available.
As illustrated above, instant dynamic channel selection is an
advantageous feature of the present invention. The instant dynamic
channel selection of the present invention enables, on average,
channel reuse at a shorter geographical distance than fixed channel
allocation (FCA) can provide for the same specific blocking
probability, whereby the locally available interference limited
traffic capacity is increased compared to fixed channel allocation
(FCA). However, it should be understood that aspects of the present
invention can performed using fixed channel allocation (FCA)
instead. Thus, features of the present invention which do not
necessarily require instant dynamic channel selection are not to be
construed as requiring instant dynamic channel selection.
Although various duplex schemes have been shown, in accordance with
the present invention each timeslot on a carrier, e.g., each
carrier/timeslot channel, can be allocated to either downlink or
uplink. With such flexibility, the TDD mode can be adapted to
different environments and different development scenarios. In any
configuration, at least one time slot on one carrier has to be
allocated for the downlink and at least one time slot on th e
carrier has to be allocated for the up link.
While the invention has been described in connection with what is
presently considered to be the most practical and preferred
embodiment, it is to be understood that the invention is not to be
limited to the disclosed embodiment, but on the contrary, is
intended to cover various modifications and equivalent arrangements
included within the spirit and scope of the appended claims.
* * * * *